URL-decode S3 object keys from event notifications before GetObject

[ngoyal16]

Problem

In main.go, S3 object keys are extracted directly from S3 event notifications and passed to GetObject:

key := record.S3.Object.Key
...
getObjectOutput, err := s3Client.GetObject(ctx, &s3.GetObjectInput{
    Bucket: aws.String(bucket),
    Key:    aws.String(key),
})

According to the official AWS S3 Event Notification specification:

The key value in an Amazon S3 event notification is URL-encoded. You must URL-decode the key before using it in Amazon S3 API calls.

For example:

• Spaces in keys are encoded as + or %20
• Characters like =, :, @, & are percent-encoded (common in partitioned S3 paths, e.g. year=2026/date=2026-09-29/)

Because the key is not unescaped, S3 returns a NoSuchKey 404 error when fetching objects with any encoded characters.

Proposed Solution

Decode the object key using Go's net/url package before calling S3 APIs:

decodedKey, err := url.QueryUnescape(key)
if err != nil {
    return fmt.Errorf("failed to decode S3 object key %q: %w", key, err)
}

Note that S3 encodes spaces as +, which url.QueryUnescape handles properly.

Acceptance Criteria

• Object keys containing URL-encoded characters (spaces, +, %20, =, etc.) are properly decoded before calling s3Client.GetObject
• Unit test covers keys with spaces, +, and special characters